You've probably heard the story about how the more varieties of jam are on display, the less likely people are to buy any jam at all. Iyengar conducted that research, as well as another experiment showing that toddlers who are told they can choose from a wide variety of toys have less fun than those who are just given one to play with. Her book is a fashionable mix of biology, business and psychology research guided by an unusually reflective and humane intelligence. Moving from appalling studies on rats and dogs to the nuances of arranged marriage, how different cultures value choice, the reasons why people prefer Coca-Cola, or colour "prediction" in the fashion trade, Iyengar tells a complex story refreshingly free from the Gladwellian genre's usual vice of monothematic overstatement.

Choice, then, "is not an unconditional good". (Iyengar mentions the supermarket Monstromart in The Simpsons, slogan: "Where shopping is a baffling ordeal".) A few ways are suggested of improving decision-making when forced to choose (crowdsource recommendations; write a Ben Franklin-style list of pros and cons), and Iyengar recommends rehearsing hard choices (eg about the care of elderly relatives) before having to confront them. There is a suggestive pause, too, to consider Ostalgie in the former communist countries – as one Polish man tells the author: "I do not need 10 types of chewing gum."

This book, by contrast, is an object lesson in the perils of trying to herd disparate facts under the roof of a single snappy concept. Dutton cites many of the same studies as Iyengar (learned helplessness in dogs, "framing" and "priming" effects, various cognitive biases), but this time to sell an idea about how to influence other people. His unlovely neologism, "flipnosis", remains an obscure concept: it is "a brand new kind of influence", but somehow also "ancient"; for much of the book it means the kind of instant jokey retort that can defuse a potentially unpleasant situation, yet Dutton also discusses the chops of salespeople and lawyers, whose mind-games operate over a longer time-scale.

The most interesting stories here are the author's own vividly reported encounters with conmen or psychopaths. The latter, Dutton insists, are not all Hannibal Lecters, but often charismatic types working in normal careers. I began to be sure that I had worked with a few.

Choose your favourite metaphor: life is a) a series of choices; b) a struggle to influence others; or c) a race or competition in which you want to be a "winner", because otherwise you are a loser. The authors of this book, a neuroscientist and a psychologist, alternate reports of brain studies with inspirational human-interest anecdotes, promising to reveal how you can change your brain to be more like these people. The "winners" interviewed include, refreshingly, a window-cleaner and a black-cab driver as well as bankers and marketeers; but the authors cheerily admit that none of their interviewees has been subjected to an fMRI scan, so the inferences about how they are using their brains remain ("probably", "presumably") highly speculative.

Eventually all the snazzy science culminates in a set of recommendations for brain health that will surprise few. Exercise more, eat fish, drink coffee. "Lonely? Make a lunch date with a friend." Oh, professors, with this scientific self-help you are really spoiling us.